The density is a measure of a mediums mass per unit of volume. The SI unit of density is the kilogram per cubic meter (kg/m3). As examples, at standard temperature and pressure condition, the density of water is 1000 kg/m3, of ethyl alcohol is 790 kg/m3, respectively.
Viscosity is a measure of the resistance of a fluid to deformation under shear stress. Viscosity describes a fluid internal resistance to flow and may be thought of as a measure of fluid friction. The SI physical unit of dynamic viscosity is the Pascal second (Pa·s). The cgs physical unit for dynamic viscosity is the poise (P). It is more commonly expressed, particularly in ASTM standards, as centipoise (cP). As example, water has a viscosity of 1.0 cP (at 20° C.).
It is known from patent document EP0282251, a transducer for measuring density or viscosity of fluid. The transducer comprises two parallel tines extending from a common yoke, forming a tuning fork adapted to be immersed in the fluid. The tines are excited to vibrate resonantly and in anti-phase by one or more piezoelectric exciting elements, which are housed in one or more cavities within the tines or the yoke. The vibrations are sensed by one or more similarly-housed piezoelectric sensing elements.
This transducer has a typical size of several centimeters requiring a bulky and expensive sensor housing. This leads to a highly intrusive measuring device preventing its use in small sample chambers or small pipes. Further, operation at high pressure and high temperature is also very difficult to implement due to the large size.